The large nut which holds the clutch bell to its bevel gear is a normal right hand thread. The trick is to hold the sleeve which the nut screws onto in place - you'll need a slim open ended spanner inserted through the hole for the kickstart bevel gear, held onto the two flats on the sleeve itself, to prevent it from turning whilst you undo the nut. It's also a good idea to wrap the shank of the spanner with insulating or gaffer tape, to stop it marking the alloy around the kickstart aperture.
If by any chance you're talking about the 14mm nut on the very front of the splined block which the clutch friction plates fit onto, that has a left-hand thread - unless yours is an early model, when you might just find it's right-handed.